Hi, I have weighed the object 300g which does seem quite heavy for an object that size. I also checked if it conducted electricity with a multimeter, it does not. Looking online I did see an image of a pyrite nodule which looks fairly similar. If this is what it is, is there anything I should do with it... cut it open, polish it, or simple stick it in the garden.

We'll get a better idea (still at the non-destructive stage) knowing its density. Calculate its volume using water displacement (I don't think there will be much error due to absorption), then let us know the density please.